Fix terraform-docs Git commit message (#45)

* To use environment variables in the action parameters, `env.` needs to
  be used rather than a shell variable (wg.g `$VARIABLE`)

.github/workflows/continuous-integration-terraform.yml

@@ -97,4 +97,4 @@ jobs:
           output-method: inject
           git-push: true
           ref: ${{ github.event.pull_request.head.ref }}
-          git-commit-message: "$GIT_COMMIT_MESSAGE - Terraform Docs"
+          git-commit-message: "${{ env.GIT_COMMIT_MESSAGE }} - Terraform Docs"